Process for reducing metal ions in aqueous solutions

ABSTRACT

Metal ions are reduced from a higher to a lower oxidation stage using ferrophosphorus as the reductant.

The present invention relates to a process for reducing metal ions from a higher to a lower oxidation stage with the use of a reductant in an aqueous solution.

It is generally known that metals which in aqueous solution may exist in form of ions of various oxidation stages can be reduced to a lower oxidation stage by means of a reductant.

The reductants which are commonly used include iron, e.g. in form of iron turnings or borings, zinc, aluminum, sulfides, sulfites, hydrazine etc.

A disadvantage of these known reductants resides in the fact that their efficiency depends on the acidity of the respective solution and that the metals give rise to the evolution of hydrogen.

We have now unexpectedly found that these adverse effects do not appear when ferrophosphorus is used as the reductant.

In order to increase the reduction velocity, it is good practice to use the ferrophosphorus in form of particles with a size smaller than 100 μm, preferably smaller than 10 μm. It is also possible to increase the reaction velocity by increasing the reaction temperature to more than 60° C.

A further technically beneficial effect resides in that the reduction can be carried out not only in an acid but also in a neutral or alkaline medium, even at a pH of 9 or more.

It is also an unexpected result which would not have been foreseen that ferrophosphorus permits a low redox potential of less than -150 mv to be established in crude phosphoric acid (45-50% P₂ O₅), and that hydrogen is practically not evolved within the acid range where commonly employed standard reductants, e.g. iron turnings or borings or hydroxymethanesulfinate, have a redox potential of +170 mv as the end point.

The reduction in accordance with this invention can be effected, for example, in a hydrochloric, sulfuric or phosphoric acid medium.

The process of this invention can be used for reducing higher valence metal ions, e.g. ions of iron, vanadium, chromium or uranium alone or in admixture with each other or further ions.

The following Examples are intended to illustrate the process of this invention which is naturally not limited thereto.

    __________________________________________________________________________        Initial redox                                                                         Conc. of metal ions                                                                      Aqueous                                                                               Reductant in                                                                              Redox potential                                                                        Conc. of reduced                 Ex.                                                                               potential                                                                             in feed material                                                                         solution                                                                              wgt %      after reduction                                                                        metal ions                       __________________________________________________________________________     1  +830 mV                                                                               Fe.sup.3+ 0.331%                                                                         HCl 12%                                                                               Ferrophosphorus                                                                           -130 mV Fe.sup.2+ 0.607%                           V.sup.5+ 426 ppm 0.8%               Fe.sup.3+ 0.013%                           Cr.sup.6+ 389 ppm                   V.sup.5+ not detectable                                                        V.sup.4/3 439 ppm                                                              Cr.sup.3+ 457 ppm                2  +618 mV                                                                               V.sup.5+ 329 ppm                                                                         H.sub.2 SO.sub.4 75%                                                                  Ferrophosphorus                                                                           +280 mV V.sup.5+ not detectable                                     0.8%               V.sup.3/4  314 ppm               3  +490 mV                                                                               V.sup.5+ 640 ppm                                                                         crude H.sub.3 PO.sub.4                                                                Ferrophosphorus                                                                           -180 mV V.sup.3+ 620 ppm                           Cr.sup.6/3 589 ppm                                                                       65%    0.8%               V.sup.5+ not detectable                    Fe.sup.3+ 0.28%                     Cr.sup.6+ not detectable                                                       Fe.sup.2+ 0.61%                                                                Fe.sup.3+ < 0.003%               4  -384 mV                                                                               V.sup.5+ 200 ppm                                                                         Na.sub.2 HPO.sub.4                                                                    Ferrophosphorus                                                                           -780 mV Fe.sup.3+ V.sup.5+                         Fe.sup.3+ 200 ppm                                                                        36% pH˜8.9                                                                      0.05% < 10 μm   not detectable                   5  +475 mV                                                                               Fe.sup.3+ 0.225% +                                                                       crude H.sub.3 PO.sub.4                                                                Na--hydroxymethane-                                                                       +220 mV Fe.sup.3+ 0.013%                           V + Cr    66%    sulfinate 0.8%                                      __________________________________________________________________________ 

We claim:
 1. A process for reducing metal ions from a higher to a lower oxidation stage by means of a reductant in an aqueous solution, which comprises using ferrophosphorus as the reductant.
 2. The process as claimed in claim 1, wherein the ferrophosphorus substantially consists of particles having a size of less than 100 μm.
 3. The process as claimed in claim 2, wherein the ferrophosphorus substantially consists of particles having a size of less than 10 μm.
 4. The process as claimed in claim 1, wherein the reduction is effected at a temperature higher than 60° C.
 5. The process as claimed in claim 1, wherein the reduction is effected in an acid, alkaline or neutral medium. 